Currently what is known regarding recovery processes in steelworks powder, comes down basically in the hydrometallurgical process, which has been considered environmentally friendly, producing residues having safe disposal, since zinc can be dissolved leaving other impurities in the residue. Sulfuric acid and ammoniacal solutions are considered to be effective reagents for waste treatments for zinc recovery. Ammoniacal solutions should not be present when the final product of interest is zinc sulfate solution. Sodium hydroxide is selective for zinc; however the metal recovery process needs to be enhanced through the electrolysis process, which leads to an overhead of energy for zinc recovery. Furthermore, it would be necessary later to attack with sulfuric acid to obtain the zinc sulfate solution, increasing the cost for reagents. The use of hydrochloric acid is not commercially viable since their attack is not selective and cost of materials is high. Organic acids are not cost-effective and do not have selectivity to iron, extracting it together.
Articles published in the journal J Hazard Mater. 2010 Jul. 15; 179 (I-3): I-7 and J Hazard Mater. 2010 Jul. 15; 179 (I-3): 8-14 reported the development of an integrated process for the recovery of zinc from electric arc furnace dust. It shows the process of leaching with dilute sulfuric acid, and purification of leach liquor and the recovery of zinc by solvent extraction and electro deposition. The leaching process was based on zinc extraction with dilute sulfuric acid under atmospheric conditions and without using any pre-treatment. The disadvantage is the use of electrolysis, which demands great energy expenditure.
The Brazilian patent PI0801716-6A shows the chemical process for treating industrial waste steel, named generically as “steelworks powder”, to enable the subsequent recovery of zinc, iron and other metals of interest herein. The main objectives of the process are: (i) the opening of crystal structures called zinc ferrites, originally formed in steelworks' powder, with the simultaneous production of soluble sulfates in aqueous media, and (ii) the destruction of possible organic chains containing chlorine (dioxins) without its subsequent regeneration. Generally, the process can be summarized as the production of sulfates, or salts of the constituent metals from the steelworks powder using concentrated sulfuric acid and reaction reagents or dry process catalysts at normal pressure and moderate temperatures. It is to promote the direct reaction of concentrated sulfuric acid with the constituent elements of steelworks' powder maintaining a homogeneous mixture without the presence of water, and preventing agglomeration of particles through addition of reagents and or secondary catalysts; in the specific case, milled solid potassium chloride. The disadvantages consist in the use of temperatures above 100° C., which facilitates joint dissolution of iron and increases energy expenditure of the process.
U.S. Pat. No. 4,610,721 is directed to a process provided for the selective recovery of zinc from steel powder containing substantial amounts of iron. The process includes atmospheric leaching of steel powder, in a first phase in which a quantity is mixed with an acidic zinc sulfate solution to leach zinc, the leaching of powder is such that the pH of the solution is controlled at 2 to 3.5 and preferably about 2.5 to 3.5, thereby limiting the dissolution of iron. The mixture is then subjected to a separation step in unprocessed/solid liquid, in which a thick pulp is obtained containing zinc and iron, and a distinct liquid containing reduced iron and substantial amounts of zinc, this is the solution being delivered to the zinc recovery. After separation, the thick pulp, together with additional or spent sulfuric acid electrolyte, it is subjected to the high pressure leaching at a temperature at least sufficient to leach remaining amounts of zinc and provide an acid zinc sulfate solution of itself, during the conversion of hydrated iron oxides into sedimented hematite residue. An additional amount of sulfuric acid is added to a foresaid thick pulp, sufficient to obtain the quantitative dissolution of zinc without iron excessive dissolution. The disadvantage is that, besides using zinc sulfate solution to dissolve it inserting zinc in the process, the reaction is not controlled stoichiometrically.
The processes currently used for recovery of zinc of waste of steelworks powder present several disadvantages, inconveniences and limitations, such as low selectivity to zinc, with co-dissolution of iron, high temperature dissolution, use of many reagents for recovery, which increase the cost of the process and the amount of effluents, the use of electrolysis to assist in dissolution, which increases the energetic demand of the process.